1. Technical Field of the Invention
The present invention relates to a method and an apparatus for visually inspecting a transparent body and a translucent body such as a glass hard disk substrate, an LCD substrate or a lens. More particularly, the present invention relates to a method and an apparatus for visual inspection in which scratches or pits which are concave defects and particles which are adherents are detected so as to be distinguished from each other.
2. Prior Art
As a conventional visual inspection method for inspecting concave defects and adherents of an object to be inspected, Japanese Laid-open Patent Application No. Sho 60-219542 discloses a method in which, using the fact that p-polarized components are included in light scattered from foreign substances, adherents on a wafer are detected by detecting p-polarized components included in scattered light by applying s-polarized laser light onto the surface of the wafer at a low angle. This method is for opaque bodies having a high surface reflectance such as wafers, and resolution is not high for transparent bodies and translucent bodies.
Japanese Laid-open Patent Application No. Hei 1-96537 discloses a method in which a laser beam is applied to the surface of a glass disk in a vertical direction, and light scattered from protruding defects existing on the glass disk is detected by a low-angle light receiver and scattered light reflected from concave defects on the glass disk to converge is extracted by a half mirror and received.
Japanese Laid-open Patent Application No. Hei 5-52764 discloses a method in which obliquely incident light is applied to a glass substrate and is received at two points of a high angle and a low angle as first detection light and second detection light, and discrimination between concave defects and adherents on the glass substrate is performed based on the intensity of the p-polarized components in the first detection light and the ratio of the intensity of the second detection light to the intensity of detection light of a first detection component.
Japanese Laid-open Patent Application No. Hei 6-138045 discloses a method in which detection light is applied to the surface of a glass substrate in a vertical direction and light scattered from fine particles existing on the obverse and the reverse surfaces of the substrate is detected by a detector disposed within the vicinity of the critical angle of total reflection of the substrate on the obverse surface of the substrate.
Japanese Laid-open Patent Application No. Hei 6-222013 discloses a method in which scratches and small particles are detected by the dark-field reflection image of a first light beam of a low incidence angle, stains are detected by the bright-field reflection image of a scattered second light beam, and pits and bulges are detected by the bright-field reflection image of a third light beam of a high incidence angle.
These methods all use reflection light and are effective in detecting adherents. However, when the object to be inspected is a transparent or a translucent body, these methods are unsuitable for detecting concave defects because contrast cannot be obtained.
As a method using transmitted light, Japanese Laid-open Patent Application No. Hei 7-225198 discloses a method in which laser light is applied to a glass substrate, and adherents are detected by a sensor disposed on the optical path of the transmitted light and concave defects are detected by a sensor disposed in a position off the optical path of the transmitted light. However, in the method using only transmitted light, not only adherents but also concave defects are simultaneously detected, so that it is difficult to distinguish concave defects from adherents.
The present invention is made in view of the above-described problems, and an object thereof is to provide a method and an apparatus for visually inspecting a transparent body and a translucent body in which concave defects and adherents are distinguished from each other and highly sensitive detection can be performed for both.
To solve the above-described problems, in the method of visually inspecting a transparent body and a translucent body according to the present invention,
light is applied to a reverse surface of an object to be inspected by a transmission light source,
light is applied, at a low angle, to at least one of an obverse surface and the reverse surface of the object to be inspected by a reflection light source, and
detection of concave defects and adherents of the object to be inspected by use of transmitted light emitted from the transmission light source and transmitted by the object to be inspected and detection of adherents of the object to be inspected by use of reflected scattered light emitted from the reflection light source and irregularly reflected at the object to be inspected are performed by a detector disposed on an obverse surface side of the object to be inspected in a position substantially parallel to an optical path of the transmission light source and where light from the transmission light source is not directly incident.
According to this method of the present invention, since concave defects and adherents of the object to be inspected are detected by use of transmitted light emitted from the transmission light source and transmitted by the object to be inspected and adherents of the object to be inspected is detected by use of reflected scattered light emitted from the reflection light source and irregularly reflected at the object to be inspected, concave defects and adherents can be inspected so as to be distinguished from each other. That is, by identifying the positions of concave defects and adherents by use of the transmission light source and then detecting adherents by use of the reflection light source, concave defects can be distinguished from adherents. Although the detection by use of the reflection light source is dark-field illumination and slightly low in contrast, since the positions of the defects are previously identified by the detection by use of the transmission light source, it is necessary only to determine whether the defect at each position is an adherent or not. Thus, the defect of low contrast can be made up for.
At the time of the detection by use of the reflection light source, scattering due to adherents on the object to be inspected is dominant, so that only the adherents can effectively be detected. Moreover, since the detector is disposed in a position not affected by total reflection, detection can be performed with high sensitivity. At the time of the detection by use of the transmission light source, since the detector is disposed in a position where light from the transmission light source is not directly incident although being close to bright-field illumination, a dark-field image with excellent contrast is obtained, so that concave defects and adherents can clearly be detected.
According to the method of the present invention, the concave defects and the adherents of the object to be inspected may be distinguished from each other by illuminating the object to be inspected with a combination of turning on and off of the transmission light source and the reflection light source and separately performing the detection of the concave defects and the adherents by use of the transmission light source and the detection of the adherents by use of the reflection light source. By doing so, the detection of concave defects and adherents by use of the transmission light source and the detection of adherents by use of the reflection light source can more clearly be performed. In this case, it is preferable to perform the detection of adherents by use of the reflection light source for both of the obverse and the reverse surfaces by turning the object to be inspected from side to side. Thereby, the adherents on the obverse and the reverse surfaces can be distinguished from each other.
Moreover, according to the method of the present invention, the reflection light source may be disposed on each of the obverse surface side and the reverse surface side of the object to be inspected, and the concave defects and the adherents of the object to be inspected may be distinguished from each other by illuminating the object to be inspected with a combination of turning on and off of the transmission light source and the two reflection light sources and separately performing the detection of the concave defects and the adherents by use of the transmission light source, detection of adherents on the obverse surface by use of the first reflection light source and detection of adherents on the reverse surface by use of the second reflection light source. By doing so, the adherents on the obverse and the reverse surfaces can be distinguished from each other without the object to be inspected being turned from side to side.
To solve the above-mentioned problems, the apparatus for visually inspecting a transparent body and a translucent body according to the present invention comprises:
a transmission light source disposed on a reverse surface side of an object to be inspected and applying light to a reverse surface of the object to be inspected;
a reflection light source disposed on at least one of an obverse surface side and the reverse surface side of the object to be inspected and applying light to the object to be inspected at a low angle; and
a detector disposed on the obverse surface side of the object to be inspected in a position substantially parallel to an optical path of the transmission light source and where the light from the transmission light source is not directly incident, detecting concave defects and adherents of the object to be inspected by use of transmitted light emitted from the transmission light source and transmitted by the object to be inspected, and detecting adherents of the object to be inspected by use of reflected scattered light emitted from the reflection light source and irregularly reflected at the object to be inspected.
By performing a visual inspection of a transparent body and a translucent body by use of this apparatus, concave defects and adherents can be inspected so as to be distinguished from each other as described above.
In the apparatus of the present invention, it is preferable that an angle of inclination of the detector from the optical path of the transmission light source be 5xc2x13 degrees. Moreover, it is preferable that an angle of incidence of the reflection light source on the object to be inspected be approximately 4xc2x13 degrees.
Moreover, in the apparatus of the present invention, a light intercepting plate for preventing the light from the transmission light source from being directly incident on the detector can be disposed between the transmission light source and the object to be inspected or between the object to be inspected and the detector. By doing so, light from the transmission light source is not directly incident on the detector, so that resolution improves.
Further, in the apparatus of the present invention, it is preferable that the transmission light source be a light source in which angles of light beams illuminating the object to be inspected are substantially the same. With this, the scattered light can be made incident on the detector without any loss.
Other objects, features, and advantages of the invention will become apparent from the following description and from the claims.